1. Field of the Invention
The present invention relates generally to electrical medical leads, and more specfically relates to transvenous cardioversion leads. For purposes of this application, "cardioversion" is used in its broad sense, as including defibrillation and termination of tachycardias.
2. Description of the Prior Art
Research to provide an automatic implantable standby cardioverter has been in progress for over ten years. One result of this research has been the development of transvenous cardioversion leads. The primary perceived advantage of a transvenous lead used in conjunction with an implantable cardioverter is that the energy required for cardioversion by this method is significantly reduced, making an implantable defibrillator feasible.
Mirowski, et al in U.S. Pat. No. 3,942,536 teaches an early defibrillation lead. The lead is provided with multiple electrodes for location at the apex of the right ventricle and multiple electrodes for location in the superior vena cava.
Rubin, in U.S. Pat. No. 3,857,398 teaches a transvenous defibrillation lead having a first electrode at the tip of the lead for location in the ventricular apex, a second electrode slightly proximal to the first electrode for location in the ventricle, and a third electrode for location in the right atrium. The first and second electrodes are used for cardiac pacing while the first and third electrodes are used for defibrillation.
In addition to benefiting the designers of implantable cardioverters, the lower energy levels associated with transvenous cariodversion also benefit the patient. As noted in the Mirowski patent referenced above, transvenous defibrillation has several medical advantages over external defibrillation as presently practiced. First, high energy transthoracic defibrillation generally results in extreme pain, often necessitating the use of general anesthetic. Second, because of the high energy levels involved, transthoracic defibrillation poses a substantial risk of cardiac damage. Both of these problems can be reduced by the precise localization of defibrillation energy allowed by the use of a transvenous defibrillation lead.
The leads disclosed by Rubin, Mirowski and others are suitable for use with low-energy standby defibrillators which, typically, generate only a fraction of the energy produced by standard external defibrillation units. However, at present, such low-energy standby defibrillators are not widely available. Use of a lead as disclosed by Rubin or Mirowski with an external defibrillation unit is impractical. First, a lead as disclosed by Rubin or Mirowski is a direct, low-impedance electrical pathway to the heart. Connection of such a lead to an external defibrillation unit poses a significant risk of injury to the patient. Typical external defibrillation units are capable of delivering up to 400 joules of energy, and any missetting or malfunction of such a device connected to a transvenous lead according to the prior art poses a significant risk of injury or death. Second, the external unit is provided with a scale which typically reads from 0 to 400 joules while transvenous defibrillation generally requires less than 40 joules and termination of tachycardias generally requires less than 2 joules. Any adjustment of the external unit must therefore take place within the lower 10 % of the scale, making accurate setting of energy levels extremely difficult. Finally, external defibrillation devices are generally designed to deliver pulses across a nominal load of 50 ohms, while the typical transvenous lead exhibits a nominal load of approximately 100 ohms.